This invention relates to a fuel injection system for use with an internal combustion engine, and more particularly to a fuel injection system which is adapted to control the valve opening pressure of the nozzle needles of the fuel injection valves by means of the pressure of fuel leaked from the valves.
Conventionally, a fuel injection system has been proposed, e.g. by U.S. Pat. No. 4,213,434, which is arranged such that fuel leaked from a fuel injection valve is stored in a drain passage leading to a lower pressure zone in a manner that the stored fuel acts upon the nozzle needle of the valve as back pressure. The amount of leaked fuel draining to the lower pressure zone is controlled as a function of one or more operating parameters of an associated engine, so as to vary the back pressure acting upon the nozzle needle to thereby control the valve opening pressure of the fuel injection valve.
On the other hand, it is known that the injection rate, i.e. injection quantity per unit period of time can be reduced, or the injection period can be lengthened in a low engine speed region, by setting the valve opening pressure of the fuel injection valve at a lower value in the low engine speed region, to thereby prevent knocking due to an ignition lag in an engine cylinder as well as combustion noize due to sudden explosive combustion in an engine cylinder lasting for a short period of time, while in a high engine speed region, the injection rate can be elevated by setting the valve opening pressure at a higher value, to obtain good output characteristics of the engine and also prevent the occurrence of nitrogen oxides (NOx).
However, according to conventional arrangements utilizing leaked fuel pressure for control of the valve opening pressure of fuel injection valves as aforementioned, the amount of fuel leaked from a fuel injection valve is relatively large in a low engine speed region, whereas in a high system speed region, the amount of the leaked fuel is relatively small, resulting in insufficient valve opening pressure. The engine therefore cannot achieve good output performance in the high speed region. Particularly, when the engine is rapidly accelerated from a lower speed region to a higher speed region, the back pressure acting upon the nozzle needles cannot be quickly increased to such a high level as to increase the valve opening pressure to a sufficient level, which renders it impossible for the engine to achieve good accelerating performance.
It is a general tendency with an ordinary fuel injection pump that there occurs a gradual increase in the fuel injection quantity as the engine rpm increases, provided that the control rod of the pump remains constant in position. This tendency is intensified by the above-mentioned low valve opening pressure of the fuel injection valve in a high engine speed region, making it the more difficult to control the fuel injection quantity with accuracy.